“The IQ of an average soccer player is about the same as that of an average person, which stands at 100 .”
[from: Smartest Footballer: A list of soccer players with the highest IQ]
“ELITE SOCCER PLAYERS are smarter than you are, and the sharpest of them score more often than dimmer teammates.”
[from: Elite Soccer Players are Smarter Than You Are - Wired Magazine]
and, finally:
"Gerard Pique-IQ score: 170
For one of the only four players to ever win a UEFA Champions League twice, one would only mistake Gerard Pique for only being a football powerhouse, but according to his father, who spilled the tea to La Vanguardia, this Spanish star had an IQ score of 170."
[from, again: Smartest Footballers]
So, on average, they’re average. And the most intellectually gifted among them? Way above average.
.
In any case, the ‘benefits of exercising’ point is not about IQ. It’s about more efficient information processing. This will generally be applied in those areas prioritized by the person in question. No matter your dispositional IQ, maintaining an active (non-exhausted) life will enable you to perform closer to your personal best.
But, of course, your overall point is accurate. I’m unaware of any natural phenomenon that does not, ultimately, have diminishing returns when applied excessively.